In the past few decades, the automotive industry has striven to expand both the number and type of vehicular functions and systems subject to computer control. Due in part to the proliferation of such computer control, however, available physical space within a vehicle has correspondingly diminished, thereby resulting in a demand for more compact control systems. At the same time, owing both to the competitive climate within the industry and to the crucial nature of some of the vehicular functions under computer control, the overall reliability of such control systems has risen to the level of paramount importance.
As an example of one such system subject to computer control, a modern automotive ignition system typically includes an ignition coil and a coil current switching device responsive to an ignition, or "drive", signal to energize the ignition coil. Some type of control circuitry, responsive to microprocessor control, provides a drive signal to the coil current switching device to thereby energize the primary side of the ignition coil.
Typical prior art automotive ignition system have incorporated the control circuitry and coil current switching device into a single ignition module using a combination of integrated circuits and passive discrete components. This combination of components has been implemented using so-called hybrid electronics technology. Essentially, hybrid electronics is an amalgamation of integrated circuit technology and discrete electronic component technology arranged and surface mounted on a ceramic substrate such as, for example, alumina.
Hybrid ignition modules have been well received in the automotive industry, but they suffer from several inherent drawbacks. First, due simply to the number and size of discrete and integrated components required for operation, the overall size of an ignition module can be quite large as compared to a typical packaged integrated circuit. This problem is compounded by limitations inherent in hybrid processing technology such as large conductor line widths and conductor routing limitations. The size and number of componentary further adds to the overall weight of the module which, as the number of such vehicular control systems increases, can become a significant factor in system design. Second, such hybrid modules are typically expensive to produce, particularly when compared to processing costs associated with comparably complex integrated circuits. Further, because of the number of module components and interconnections therebetween, module reliability can be significantly less than that of comparably complex integrated circuits.
Designers of automotive ignition modules have attempted to overcome the foregoing drawbacks inherent in hybrid technology by designing so-called "all silicon" ignition coil driver modules. Such circuits use only integrated circuits and no passive discrete components. Prior art implementations of all silicon ignition coil driver modules typically derive their power from the control signal provided by an engine control computer or other microprocessor-based control computer. For example, when the computer-generated control signal transitions to an "on" state, the voltage provided to the ignition coil driver module energizes its circuitry and turns on the coil primary current. Conversely, when the computer-generated control signal transitions to an "off" state, the voltage removed from the ignition coil driver module deenergizes its circuitry and turns off the coil primary current which, in turn, generates an ignition spark. This type of system mechanisation eliminates the need for a connection from the ignition coil driver module to the vehicle battery, and an example of a system employing this concept is described in U.S. Pat. No. 5,781,047 to John R. Shreve et al., which is assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference.
Implementations of all silicon ignition coil driver modules that are powered from a control computer have several drawbacks associated therewith. For example, such systems are only suitable for limited function ignition systems. Any ignition function that requires the module to be powered after the coil current has been turned off cannot be implemented in this manner. As another example, powering the ignition coil driver module from a control computer places additional demands on the computer's output circuitry to provide the needed voltage and current required by the ignition coil driver module.
To overcome the foregoing limitations, an all silicon ignition coil driver module is needed wherein such a module is powered from an independent power source, such as for example, the vehicle battery. In such a system, however, care must be taken to protect the integrated circuit from transient and other fault conditions typically associated with an external voltage supply. Throughout the automotive industry, for example, integrated circuitry having battery voltage supply inputs thereto are typically subject to specified voltage transient requirements. One such requirement is referred to "reverse battery" wherein the battery's polarity is reversed and the normally positive voltage supply input of the ignition coil driver circuitry is made negative, and the normally negative voltage supply input (e.g., ground) is made positive. A test for this requirement is typically performed with the equivalent of fully charged 12-volt automotive battery. The ignition coil driver circuitry must survive this test without overheating to the point of destruction or drawing such a large current that its interconnect wiring is fused open. Another voltage transient requirement is commonly referred to as a "load dump" wherein a large positive voltage ranging typically between 40-80 volts is applied to the voltage supply input of the ignition coil driver circuitry. As with the reversed battery requirement, the ignition coil driver circuitry must survive the load dump test without overheating to the point of destruction or drawing such a large current that its interconnect wiring is fused open. Yet another voltage transient requirement is that the voltage supply input of the ignition coil driver circuitry must be robust to electrostatic discharge (ESD) up to +/-25 kV using the commonly specified human body model (HBM). Yet another voltage transient requirement is that the voltage supply input of the ignition coil driver circuitry must be robust to an arc from the secondary of the ignition coil itself. Although the voltage arc requirement is similar to the ESD requirement, the energy is substantially higher.
What is therefore needed is a transient protection circuit that is operable to satisfy each of the foregoing voltage transient requirements, yet is small and simple enough in structure to be integrated into an application specific monolithic integrated circuit.